Movable arms on which equipment may be mounted are known in the art such as seen in U.S. Pat. No. 3,030,128 issued Apr. 17, 1962 to K. Versen. The movable arm shown in this patent utilizes three rotating and swivel joints in conjunction with friction couplings and a torsion spring to counterbalance the weight of a lamp at the end of the arm. The interaction of all these joints and elements permits the lamp at the end of the movable arm to be easily positioned in a large number of positions within the reach of the arm, and the lamp will stay in the position in which it is placed.
There are, however, problems with such prior art movable arms. Each movable joint has only two degrees of freedom, and when it is desired to orient the lamp or other equipment in a specific position, there is often difficulty in that the three movable joints do not cooperatively move as easily as desired when the lamp or other equipment at the end of the arm is moved.
In addition, in some applications it is sometimes desired to change the type of equipment mounted on the end of the movable arm. In the prior art, the weight of the new equipment must be the same as the weight of the original equipment being replaced. If heavier or lighter equipment is placed on the end of the arm, the counterbalancing forces within the arm are not optimum for the new weight and the equipment sometimes will not stay in a position in which it is placed. For example, with the heavier piece of equipment the arm will sag when the equipment is manually positioned and then released. To compensate for this type of problem in the prior art, the pressure on friction coupling elements in one or more of the movable joints is increased or decreased. However, when the pressure is increased it is correpondingly harder to position the heavier equipment on the end of the arm due to jerky arm movement, with the result being difficulty in positioning the equipment in a precise position. This is caused by greater forces being required to overcome the increased friction and therefore increasing the tendency to overshoot. Thus, changing pressure on friction couplings to accommodate differing weight loads on the end of a movable arm is impractical. Accordingly, in the prior art a movable arm is usually designed only for a given weight load on the end of the arm. This has been acceptable in the prior art as there has not been much demand for movable arms that can accommodate differing weight loads.
There is a need in the art for a movable arm that can be used with differing weight loads at the end of the arm without any change in the force required to move the load and without requiring different parts.
There is also a need in the art for a movable arm that can be moved more easily than in the prior art.